storm-surge

Storm surge

Storm surge or tidal surge is an offshore rise of water associated with a low pressure weather system, typically a tropical cyclone. Storm surge is caused primarily by high winds pushing on the ocean's surface. The wind causes the water to pile up higher than the ordinary sea level. Low pressure at the center of a weather system also has a small secondary effect, as can the bathymetry of the body of water. It is this combined effect of low pressure and persistent wind over a shallow water body which is the most common cause of storm surge flooding problems. The term "storm surge" in casual (non-scientific) use is storm tide; that is, it refers to the rise of water associated with the storm, plus tide, wave run-up, and freshwater flooding. When referencing storm surge height, it is important to clarify the usage, as well as the reference point. National Hurricane Center tropical cyclone reports reference storm surge as water height above predicted astronomical tide level, and storm tide as water height above NGVD-29.

In areas where there is a significant difference between low tide and high tide, storm surges are particularly damaging when they occur at the time of a high tide. In these cases, this increases the difficulty of predicting the magnitude of a storm surge since it requires weather forecasts to be accurate to within a few hours. Storm surges can be produced by extratropical cyclones, such as the "Halloween Storm" of 1991 and the Storm of the Century (1993), but the most extreme storm surge events occur as a result of tropical cyclones. Factors that determine the surge heights for landfalling tropical cyclones include the speed, intensity, size of the radius of maximum winds (RMW), radius of the wind fields, angle of the track relative to the coastline, the physical characteristics of the coastline and the bathymetry of the water offshore. The SLOSH (Sea, Lake, and Overland Surges from Hurricanes) model is used to simulate surge from tropical cyclones.

Mechanics

At least five processes can be involved in altering tide levels during storms: the pressure effect, the direct wind effect, the effect of the earth's rotation, the effect of waves, and the rainfall effect.. The pressure effects of a tropical cyclone will cause the water level in the open ocean to rise in regions of low pressure atmospheric and fall in regions of high pressure. The rising water level will counteract the low atmospheric pressure such that the total pressure at some plane beneath the water surface remains constant. This effect is estimated at a 10 mm (0.4 in) increase in sea level for every millibar drop in atmospheric pressure.

Strong surface winds cause surface currents perpendicular to the wind direction, by an effect known as the Ekman Spiral. Wind stresses cause a phenomenon referred to as "wind set-up", which is the tendency for water levels to increase at the downwind shore, and to decrease at the upwind shore. Intuitively, this is caused by the storm simply blowing the water towards one side of the basin in the direction of its winds. Because the Ekman Spiral effects spread vertically through the water, the effect is inversely proportional to depth. The pressure effect and the wind set-up on an open coast will be driven into bays in the same way as the astronomical tide.

The Earth's rotation causes the Coriolis effect, which bends currents to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. When this bend brings the currents into more perpendicular contact with the shore it can amplify the surge, and when it bends the current away from the shore it has the effect of lessening the surge.

The effect of waves, while directly powered by the wind, is distinct from a storm's wind-powered currents. Powerful wind whips up large, strong waves in the direction of its movement. Although these surface waves are responsible for very little water transport in open water, they may be responsible for significant transport near the shore. When waves are breaking on a line more or less parallel to the beach they carry considerable water shoreward. As they break, the water particles moving toward the shore have considerable momentum and may run up a sloping beach to an elevation above the mean water line which may exceed twice the wave height before breaking.

The rainfall effect is experienced predominantly in estuaries. Hurricanes may dump as much as 12 inches of rainfall in 24 hours over large areas, and higher rainfall densities in localized areas. As a result, watersheds can quickly surge water into the rivers that drain them. This can increase the water level near the head of tidal estuaries as storm-driven waters surging in from the ocean meet rainfall flowing from the estuary.

Surge and wave heights on shore are affected by the configuration and bathymetry of the ocean bottom. A narrow shelf, or one that has a steep drop from the shoreline and subsequently produces deep water in close proximity to the shoreline tends to produce a lower surge, but a higher and more powerful wave. This situation well exemplified by the southeast coast of Florida. The edge of the Floridian Plateau, where the water depths reach 91 meters (299 ft), lies just 3,000 m (9,843 ft) offshore of Palm Beach, Florida; just 7,000 m (22,966 ft) offshore, the depth increases to over 180 m (591 ft). The 180 m (591 ft) depth contour followed southward from Palm Beach County lies more than 30,000 m (98,425 ft) to the east of the upper Keys.

Conversely, coastlines such as those along the Gulf of Mexico coast from Texas to Florida, have long, gently sloping shelves and shallow water depths. On the Gulf side of Florida, the edge of the Floridian Plateau lies more than 160 km (99 mi) offshore of Marco Island in Collier County. Florida Bay, lying between the Florida Keys and the mainland, is also very shallow; depths typically vary between 0.3 and 2 meters (.9 and 6.6 ft). These areas are subject to higher storm surges, but smaller waves. This difference is because in deeper water, a surge can be dispersed down and away from the hurricane. However, upon entering a shallow, gently sloping shelf, the surge can not be dispersed away, but is driven ashore by the wind stresses of the hurricane. Topography of the land surface is another important element in storm surge extent. Areas where the land lies less than a few meters above sea level are at particular risk from storm surge inundation.

For a given topography and bathymetry the surge height is not solely effected by peak wind speed; the size of the storm also effects the peak surge. With any storm the piled up water has an exit path to the sides and this escape mechanism is reduced in proportion to the surge force (for the same peak wind speed) as the storm covers more area.

Measuring surge

Surge can be measured directly at coastal tidal stations as the difference between the forecast tide and the observed rise of water. This information can be viewed real-time on the NOAA Tides and Currents website, as long as the station is reporting.

Another method of measuring surge was implemented by NHC starting in 2005, with a USGS team deploying pressure transducers along the coastline just ahead of an approaching tropical cyclone. This was first tested for Hurricane Rita. This method was validated against other surge measurements taken for Rita, and was subsequently used during Ernesto in 2006. These types of sensors can be placed in locations that will be submerged, and can accurately measure the height of water above them.

After surge from a tropical cyclone has receded, teams of surveyors map high water marks (HWM) on land, in a rigorous and detailed process that includes photos and written descriptions of the marks. HWM denote the location and elevation of flood waters from a storm event. When HWM are analyzed, if the various components of the water height can be broken out so that the portion attributable to surge can be identified, then that mark can be classified as storm surge. Otherwise, it is classified as storm tide. HWM on land are referenced to a vertical datum (a reference coordinate system). During evaluation, HWM are divided into four categories based on the confidence in the mark; only HWM evaluated as "excellent" are used by NHC in post storm analysis of the surge.

Two different measures are used for storm tide and storm surge measurements. Storm tide is measured using a geodetic vertical datum (NGVD 29 or NAVD 88). Since storm surge is defined as the rise of water beyond what would be expected by the normal movement due to tides, storm surge is measured using tidal predictions, with the assumption that the tide prediction is well-known and only slowly varying in the region subject to the surge. Since tides are a localized phenomenon, storm surge can only be measured in relationship to a nearby tidal station. Tidal bench mark information at a station provides a translation from the geodetic vertical datum to mean sea level (MSL) at that location, then subtracting the tidal prediction yields a surge height above the normal water height.

Records

The highest storm tide noted in historical accounts was produced by the 1899 Cyclone Mahina, estimated at 13 meters (43 ft) at Bathurst Bay, Australia, but research published in 2000 noted the majority of this was likely wave run-up, due to the steep coastal topography. In the United States, one of the greatest recorded storm surges was generated by 2005's Hurricane Katrina, which produced a maximum storm surge on the order of 7.6 meters (25 ft) around St. Louis Bay, Mississippi, in the communities of Waveland, Bay St. Louis, Diamondhead, and Pass Christian, with a storm surge height of 8.5 m (27.8 ft) in Pass Christian. Another record storm surge occurred in this same area from Hurricane Camille in August 1969, with the highest storm tide of record noted from a HWM as 7.5 m (24.6 ft), also found in Pass Christian. Other Atlantic hurricanes that were accompanied by a large and catastrophic storm surge include hurricanes Andrew, Wilma, Gilbert, Mitch, and Ike, which produced the greatest storm surge of any Category 4 hurricane on record. The worst storm surge, in terms of loss of life, was the 1970 Bhola cyclone and in general the Bay of Bengal is particularly prone to tidal surges.

SLOSH

The National Hurricane Center forecasts storm surge using the SLOSH model, which stands for Sea, Lake and Overland Surges from Hurricanes. The model is accurate to within 20 percent. SLOSH inputs include the central pressure of a tropical cyclone, storm size, the cyclone's forward motion, its track, and maximum sustained winds. Local topography, bay and river orientation, depth of the sea bottom, astronomical tides, as well as other physical features are taken into account, in a predefined grid referred to as a SLOSH basin. Overlapping SLOSH basins are defined for the southern and eastern coastline of the continental U.S. Some storm simulations use more than one SLOSH basin; for instance, Katrina SLOSH model runs used both the Lake Ponchartrain / New Orleans basin, and the Mississippi Sound basin, for the northern Gulf of Mexico landfall. The final output from the model run will display the maximum envelope of water, or MEOW, that occurred at each location. To allow for track or forecast uncertainties, usually several model runs with varying input parameters are generated to create a map of MOMs, or Maximum of Maximums. And for hurricane evacuation studies, a family of storms with representative tracks for the region, and varying intensity, eye diameter, and speed, are modeled to produce worst-case water heights for any tropical cyclone occurrence. The results of these studies are typically generated from several thousand SLOSH runs. These studies have been completed by USACE, under contract to the Federal Emergency Management Agency, for several states and are available on their Hurricane Evacuation Studies (HES) website. They include coastal county maps, shaded to identify the minimum SSHS category of hurricane that will result in flooding, in each area of the county.

Mitigation

Although meteorological surveys alert about hurricanes or severe storms, in the areas where the risk of coastal flooding is particularly high, there are specific storm surge warnings. These have been implemented, for instance, in Holland, Spain, the United States, and Great Britain.

Another modern development (in use in the Netherlands) is the creation of housing communities at the edges of wetlands with floating structures, restrained in position by vertical pylons. Such wetlands can then be used to accommodate runoff and surges without causing damage to the structures while also protecting conventional structures at somewhat higher low-lying elevations, provided that dikes prevent major surge intrusion.